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Acta Scientiae Veterinariae, 2014. 42: 1211.
RESEARCH ARTICLE
ISSN 1679-9216
Pub. 1211
Conduction System in the Swine Heart: Essential Landmarks for Gross Dissection
Thaworn Mingsakul1, Preeyaporn Surachon1 & Reon Somana2
ABSTRACT
Background: The components of the cardiac conduction system (CCS) were discovered almost two centuries and presented
in the diagrammatic forms. This should be due to the difficulty in distinguishing the CCS from the surrounding cardiac
tissues and the lack of information concerning the precise landmarks for gross dissection. Furthermore the CCS in pig,
the animal regarded as a suitable model for the assessment of catheter based intervention, has not been reported. The aims
of the present study were to demonstrate the gross anatomic architecture of CCS in the swine heart, and to provide the
valuable landmarks for the gross anatomic dissection of the CCS.
Materials, Methods & Results: Twenty hearts of adult Large White pigs (Sus Scrofa domesticus) were used. Fifteen hearts
were elucidated by gross anatomic dissection. The tissue blocks of the sinoatrial node (SAN) and atrioventricular conduction tissue of the five hearts were prepared for histological investigation by staining with Masson’s trichrome. It was found
histologically that the cardiac conduction tissues were clearly distinguishable from the surrounding cardiac myocardium
and connective tissue. Moreover, the histological information also navigated the location and anatomical architecture of
the CCS which provided essential guideline for gross dissection. The SAN was somewhat spindle in shape which embedded in epicardial connective tissue of the terminal sulcus. In some cases, it was quite difficult to identifythe SAN, so the
sinoatrial node artery was used as a clue. The AVN was an elliptical shaped which situated in subendocardial tissue at
apex of the triangle of Koch on the atrial surface of the central fibrous body. The distal extremity of the AVN extended into
the central fibrous body to form the penetrating bundle of His. At the boundary between the membranous and muscular
parts of the interventricular septum, the AV bundle of His divided into the right (RBB) and left bundle branches (LBB).
The RBB coursed downward in the muscular part of the interventricular septum around the base of the anterior papillary
muscle and then traveled along the septomarginal trabecula. The LBB passed through the lower rim of the membranous
septum and appeared on the endocardial surface of the left ventricle at the commissure between the right coronary and
non-coronary leaflets of the aortic valve. However, intramural branches of the Purkinje network could not be identified.
Discussion: The present study reveals that the anatomic architecture and location of the CCS in the pig heart could be
demonstrated by gross anatomic dissection, and the essential landmarks for gross investigation were suggested. The crucial
landmarks for the identification of the SAN, the AV conduction tissue and the LBB are the superior cavoatrial junction,
the attachment of the septal leaflet adjoining the membranous septum, and the junction between the right coronary and
noncoronary leaflets of the aortic valve, respectively. These landmarks are useful not only for gross anatomic dissection
of the CCS in the pig heart but also provide the fundamental information for cardiac arrhythmia and sudden death experimentation in this animal. Therefore these findings should be valuable for anatomists, cardiovascular researchers and
veterinary clinicians as well as veterinary and medical students.
Keywords: cardiac conduction system, sinoatrial node, atrioventricular node, atrioventricular bundle, swine heart.
Received: 4 April 2014
Accepted: 18 September 2014
1
Published: 28 September 2014
2
Department of Anatomy, Faculty of Veterinary Medicine, Khon Kaen University, Khon Kaen province, Thailand. Faculty of Medicine, Mahasarakham
University, Mahasarakham province, 40115, Thailand. CORRESPONDENCE: T. Mingsakul [[email protected] - Fax: +66 (0)4-336-4496]. Department
of Anatomy, Faculty of Veterinary Medicine, Khon Kaen University, Khon Kaen province, 40002, Thailand.
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T. Mingsakul, P. Surachon & R. Somana. 2014. Conduction System in the Swine Heart: Essential Landmarks for Gross Dissection.
Acta Scientiae Veterinariae. 42: 1211.
INTRODUCTION
The specimens were divided into Group I (five
hearts) for light microscopic study, and Group II (fifteen hearts) for gross anatomic dissection. In Group I,
the tissue blocks of the SAN and AV conduction tissue were fixed in 10% neutral-buffered formalin for 4
days, and then were prepared according to histological
standard procedures. The sinoatrial node tissue block
was sectioned perpendicular to the long axis of the
terminal crest (Figure 3A) whereas the atrioventricular conduction tissue block was cut perpendicular to
the attachment of the septal leaflet (Figure 5A). After
staining with Masson’s Trichrome, the tissue sections
were examined and photographed under light microscope (PrimoStar, Carl Zeiss1) with AxioCam ERc5S
imaging system (Axio Vision LE)2. In Group II, the
hearts were immerged in 10% neutral formalin for 7
days before the AV conduction tissue was investigated
by gross anatomic dissection and then photographed.
Anatomical structure of the cardiac conduction system (CCS) has been introduced since nineteenth century. The Purkinje network was discovered
by Purkinje in 1839 and in 1893 the atiroventricular
bundle was demonstrated by His [22]. In 1906,
Tawara [2] identified the atrioventricular node, and
right and left bundle branches. Thereafter, Keith and
Flack [15] introduced the sinoatrial node. The CCS
components were discovered almost two centuries;
however, most of their locations and morphologies
have been displayed as the diagrammatic illustrations based on histological sections [3,9] or serial
reconstructions [1,9,22]. It is very difficult to distinguish the conduction tissues from the surrounding
cardiac tissues by gross dissection due to the lack of
information to used as critical landmarks. Although
a number of gross anatomical investigations of the
human CCS have been demonstrated [13,24], those
in animals do not presented, especially during the
gross anatomy dissection classes of small and large
animals.
It is interesting to elucidate the gross anatomical structures of the CCS in the pig heart which
regarded as a suitable animal model for the assessment
of catheter-based intervention [23] and myocardial
infarction [7]. The aims of the present study are to
demonstrate the gross anatomic architectures of CCS in
the swine heart, and to provide the valuable landmarks
to approach the CCS in gross dissection and clinical
investigation.
RESULTS
Prior histological and gross anatomic investigations of the CCS performed, the related and
helpful anatomic structures to localize the conduction tissue were identified. In figure 1 the terminal
crest was situated between the sinus venarum and
right atrial appendage. It was a vertical crest posing on endocardial aspect of the right atrium, which
corresponding to the terminal sulcus on epicardial
aspect where the SAN situated. A triangle of Koch
was also identified, because of the AVN situating at
its apex. The apex of this triangle was bordered anteriorly by the attachment of the septal leaflet of the
tricuspid valve and posteriorly by a fibrous tendon
of Todaro. Base of the triangle was confined by the
coronary sinus orifice. Moreover, the membranous
and muscular portions of the interventricular septm
as well as septomarginalis trabecula were clarified
(Figure 1).
The tissue sections stained with Masson’s
trichrome distinctly differentiated that the cardiac
myocytes, connective tissue and cardiac conduction
tissue were red, blue and pink, respectively. It was
presented that the cardiac conduction tissue was clearly
distinguishable from the surrounding cardiac myocardium and connective tissue. Moreover, the histological
information also navigated the anatomical architecture
of the CCS and its location which providing an essential data for gross dissection.
MATERIALS AND METHODS
Twenty normal hearts of adult Large White
pigs (Sus Scrofa domesticus), weighing between
90-120 kg, were used. They were obtained from the
slaughter house in Khon Kaen province, Thailand.
The cardiac chambers were opened before rinsing in
tap water to completely wash the blood out. The right
atrium and ventricle were exposed by making an incision beginning from the tip of the right atrial appendage to the inferior border of the right atrium. Then the
incision was extended through the acute margin of the
right ventricle to the apex of the heart. The left ventricle
was opened by making a longitudinal incision from the
ascending aorta and cut downward through the wall of
the left ventricle along a line slightly to the left of the
anterior interventricular sulcus to the apex of the heart.
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T. Mingsakul, P. Surachon & R. Somana. 2014. Conduction System in the Swine Heart: Essential Landmarks for Gross Dissection.
Acta Scientiae Veterinariae. 42: 1211.
In 4 from 15 hearts, the identification of the
SAN quite difficult so that the sinoatrial node artery
was used as the clue to localize them. This was a
principal arterial supply of the SAN. This artery was
originated from the proximal part of the right coronary
artery and then ran upward on the medial wall of the
right atrium and directly toward the SAN (Figure 4B).
The artery could be traced to the superior cavoatrial
junction that led to the discovery of the SAN easily.
Figure 1. Photograph of endocardial aspect of the right atrium and right
ventricle showing terminal crest (red dotted line) located between the right
atrial appendage (RAA) and the superior and inferior venae cavae (SVC
& IVC). The triangle of Koch is bounded by the attachment of the septal
leaflet, the tendon of Todaro (blue dashed lines), and the coronary sinus
(CS) orifice; IVS, interventricular septum; SMT, septomarginalis trabecula;
arrow, membranous septum.
Sinoatrial Node (SAN)
Figure 2. Micrograph of the section through the upper extremity of the
sinoatrial node (SAN), corresponding to level A in Figure 3A. The node
lying in subepicardial tissue of the terminal sulcus and enveloped with
dense connective tissue. TC, terminal crest; arrow, branch of the sinoatrial
node artery; NF, nerve fiber.
With histological investigation, the SAN was
embedded in epicardial connective tissue of the terminal sulcus (Figure 2). The individual nodal cells
were smaller and slightly paler than the atrial working
myocytes. Moreover, the node was enclosed by fibrous
connective tissue and encircled around the sinoartrial
node artery, allowing clearly distinguishable from the
surrounding non-nodal tissue. From serial sections,
the position of the SAN could be specified at lateral
border of the terminal crest which adjoining to the sinus
venarum. Furthermore, it extended from the superior
cavoatrial junction to upper portion of the sinus venarum (Figure 3A, B & C).
At macroscopic level, the superior extremity
of the SAN was recognized as a small epicardial
ridge at the junction of the superior vena cava with
the right atrial appendage. After removal of subepicardial tissue, the SAN was found to located at the
junction between the musculature of the superior
vena cava and the right atrial appendage (Figure
4A). The node was somewhat spindle in shape, and
its inferior extremity extended toward the entrance
of the inferior vena cava.
Figure 3. Photographs of the right atrial tissue block and sections of the
sinoatrial node (SAN). A: Endocardial aspect of the cavoatrial junction
showing the terminal crest (TC) situated between the sinus venarum (SV)
and right atrial appendage (RAA), black dashed lines indicate levels of
sections that made perpendicular to the long axis of the terminal crest and
pass through the SAN (magenta area). B & C: The corresponding tissue
sections at levels B and C showing the SAN lying subepicardial tissue within
the terminal sulcus and its vicinity border marked with a black dotted line.
IVC, inferior vena cava; SVC, superior vena cava.
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T. Mingsakul, P. Surachon & R. Somana. 2014. Conduction System in the Swine Heart: Essential Landmarks for Gross Dissection.
Acta Scientiae Veterinariae. 42: 1211.
Figure 4. Photograph of the right supero-lateral aspect of the heart. A: The sinoatiral node (SAN) situated between the right atrial appendage (RAA) and superior vena cava (SVC). B: The sinoatrial node artery (SNA) branching from the right coronary artery (RCA)
to supply the SAN. AO, aorta.
Atrioventricular (AV) Conduction Tissue
removal of the endocardial tissue at the apex of
Koch’s triangle, the AVN was identified (Figure
6A). It was an elliptical shaped and ensheathed by
loose connective tissue. At the apex of the triangle
of Koch corresponding to the membranous septum,
the distal extremity of the AVN penetrated the central fibrous body to form the penetrating bundle of
His (BH). Diameter of the BH was nearly constant
in width and slightly smaller than the AVN. At the
boundary between the membranous and muscular
parts of the interventricular septum, the BH divided
into the right and left bundle branches (RBB &
LBB). The RBB continued down to the right ventricular aspect of the interventricular septum. Its
initial segment was buried within the muscular part
of the interventricular. Having reached the base of
the anterior papillary muscle, it traveled along the
septomarginal trabecula (Figure 6A).
The LBB passed through the lower rim of
the membranous septum and arrived the endocardial surface of left ventricle at the commissure between the right coronary and non-coronary leaflets
of the aortic valve (Figure 6B). Then it descended
in the subendocardial tissue of the muscular part
of interventricular septum, before distributed over
the internal surface of the left ventricle as Purkinje
fibers. The LBB could be directly observed under
the ventricular endocardium as a whitish band
compared with darker underling interventricular
muscles.
The atrioventricular (AV) conduction tissue
consisted of the atrioventricular node (AVN), the
atrioventricular (AV) bundle of His, the right and
left bundle braches (RBB & LBB). The AV conduction tissue was sectioned perpendicular to the
attachment of the septal leaflet of the tricuspid valve
(Figure 5A). It was showed slightly paler staining
than the working ventricular myocytes; therefore,
it was clearly distinguishable from the surrounding
ventricular muscle. The AVN was placed in subendocardial tissue of the atrioventricular junction, on
the atrial surface of the central fibrous body (Figure
5B). Its upper border was blended with the cardiac
myocytes of the interatrial septum, whereas the lower border merged with those of the interventricular
septum. The distal extremity of the AVN extended
into the central fibrous body so-called the penetrating bundle or the atrioventricular (AV) bundle of
His (BH) (Figure 5C). This penetrating part was
distinguished from the compact AVN at the point
itself completely surrounded by the central fibrous
body. A short segment of the BH was positioned on
the summit of the muscular ventricular septum and
encircled by thin collagenous connective tissue. At
this region, the BH gave rise to the left bundle branch
and the bundle itself continued as the right bundle
branch (Figure 5D).
With gross dissection, the AV conduction
tissue was easily distinguished from the surrounding cardiac myocytes and connective tissue on the
basis of its whitish color (Figure 6A & B). After
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T. Mingsakul, P. Surachon & R. Somana. 2014. Conduction System in the Swine Heart: Essential Landmarks for Gross Dissection.
Acta Scientiae Veterinariae. 42: 1211.
Figure 5. Photograph and micrographs of the atrioventricular junction. A: The atrioventricular node (orange
area) locating at apex of triangle of Koch, black dashed lines indicate the levels of sectioned through the AV
conduction tissue. The right bundle branch (light orange) coursing through the septomarginalis trabecula (SMT).
B: The atrioventricular node (AVN) encountering at the atrioventricular junction. C:The penetrating bundle
of His (BH) depositing in the connective tissue of the central fibrous body (CFB). D: The bundle of His (BH)
branching into the right bundle branch (RBB). CS, coronary sinus; IVS, interventricular septum; MV, mitral
valve; SL, septal leaflet of the tricuspid valve; *membranous septum.
Figure 6. Photographs presenting the anatomical architectures of the cardiac conduction system. A: The bundle of His (BH) continuing from the
atrioventricular node (AVN), passing through the central fibrous body (CFB) and bifurcating into the left and right bundle branches (RBB), which
running through the septomarginalis trabecula. B: The left bundle branch (LBB) arriving the left ventricular chamber at the junction between the
right coronary and non-coronary leaflets (RCL & NCL). CS, coronary sinus; *membranous septum.
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T. Mingsakul, P. Surachon & R. Somana. 2014. Conduction System in the Swine Heart: Essential Landmarks for Gross Dissection.
Acta Scientiae Veterinariae. 42: 1211.
DISCUSSION’
left ventricular surface of the interventricular septum
as seen in human [13] and rabbit [4, 23]. As the LBB
appears in the left ventricle at the junction between the
right coronary and non-coronary leaflets of the aortic
valve, this frontier should be a crucial landmark for
its identification.
The regional anatomy of the CCS in relation
to the surrounding structures is important not only
for dissecting the CCS, but also as fundamental basis
for improving the effective and safety of the catheter
ablation of cardiac arrhythmias [8,19]. Moreover, the
septal isthmus is the target to ablate the slow pathway
in AV node reentrant tachycardia flutter and isthmus
dependent atrial flutter [20]. This area is part of the
right atrial vestibule located between the coronary
sinus orifice and the attachment of the septal leaflet of
the tricuspid valve.
The present study shows that the anatomic architecture and specific location of the CCS in the pig heart
are clearly demonstrated by gross anatomic dissection.
Furthermore, the essential landmarks providing the ability
to precisely dissect the CCS components were proposed.
Morphological variations of human SAN,
resembling a triangle, horse-shoe, ellipse, or crescent
depending on the forms of the sulcus terminalis, was
proposed by Kawashima [14]. Moreover, the SAN
in rabbit appeared as a thin layer of conducting cells
[22]. By contrast the pig SAN revealed in the present
study to be a spindle-shaped similar to that in human
[9,10,18,19]. The differences of these findings may
due to the ability of the investigation techniques. In
addition, the cardiac specialized cells of the SAN was
gradually blended with the surrounding atrial myocytes. Hence it is very difficult to specify the accurately
border of the node in gross specimens. Although the
morphology of the SAN varies among individuals, its
position is almost constant. The superior cavoatrial
junction has been a common location of the SAN in
human [3,9,10,18,19], rabbit [22] and in pig. Therefore
this area should be a crucial landmark for identification
of the SAN. Moreover, another important structure is
the sinoatrial node artery that could be used as a clue
to locate the SAN during dissection [12,24].
It is generally accepted that the AVN situates at
the apex of the triangle of Koch [1,3,5,6,14,23] which
is confined by the attachment of the septal leaflet of
the tricuspid valve anteriorly and the tendon of Todaro
posteriorly [3]. The tendon of Todaro is inconveniently
identified leading Kawashima and Sasaki [13] to
propose the boundary between the membranous and
muscular parts of the interventricular septum to be the
crucial landmark for dissection of the AV conduction
tissue. While in the present study, the septal leaflet was
absolutely identified and it will navigate the location
of the AVN. After the AVN obtained, the bundle of His
and the RBB could be conveniently discovered. Thus,
the worthy landmark for exploration of the AV conduction tissue should be the attachment of the septal leaflet
adjoining the membranous septum. Moreover, the RBB
is buried in the muscular tissue of the interventricular
sepum and travels along the septomarginal trabecula
similar to that of human [16].
The LBB in the pig is clearly observed as a
whitish structure running in the subendocardium of the
CONCLUSION
The present study found that the crucial landmarks for the identification of the SAN, the AV conduction tissue and the LBB are the superior cavoatrial
junction, the attachment of the septal leaflet adjoining
the membranous septum, and the junction between the
right coronary and non-coronary leaflets of the aortic
valve, respectively. These landmarks are useful not
only for gross anatomic dissection of the CCS in the pig
heart, but also provide the fundamental information for
cardiac arrhythmia and sudden death experimentation
in this animal. It is obvious that these findings should
be valuable for anatomists, cardiovascular researchers
and veterinary clinicians, as well as veterinary and
medical students.
SOURCES AND MANUFACTURERS
1
Carl Ziess MicroImaging GmbH, Goettingen, Germany.
2
Rushmore Precision Co., Ltd., Bangkok, Thailand.
Funding. This work was financial supported by Khon Kaen
University under the Incubation Researcher Project.
Ethical approval. The study protocol has been reviewed and
approved by the Animal Ethics Committee of Khon Kaen
University, based on the Ethic of Animal Experimentation of
National Research Council of Thailand (AEKKU 08/2553).
Declaration of interest. The authors report no conflicts of
interest. The authors alone are responsible for the content and
writing of the paper.
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T. Mingsakul, P. Surachon & R. Somana. 2014. Conduction System in the Swine Heart: Essential Landmarks for Gross Dissection.
Acta Scientiae Veterinariae. 42: 1211.
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